Orthopedic joint prostheses are used to replace diseased joints, such as in the hip, knee, ankle or shoulder. An orthopedic joint prosthesis includes bearing surfaces that allow for articulation similar to that provided by the articulating surfaces of a natural joint.
One problem associated with orthopedic joint prostheses is wear of the bearing components. During articulation, the bearing surfaces slide against each other under load, which results in wear of the bearing surfaces, including loss of minute particles from the bearing surfaces. Over time, such particulates accumulate in the body of the patient, where it is theorized that the particles may cause adverse physiological reactions in some patients. Additionally, gradual loss of particulates results in erosion of the bearing surfaces, which may eventually lead to failure of the prosthesis. Various efforts have been made to minimize wear debris in joint prostheses. In recent years, efforts have focused on the use of metal-on-metal (“MOM”) joint prostheses, in which both of the articulating surfaces are metal, and ceramic-on-metal (“COM”) prostheses, in which one of the articulating surfaces is ceramic and the opposing surface is metal, and ceramic-on-ceramic (“COC”) prostheses, in which both of the articulating surfaces are ceramic.
Lower wear is expected with COC and COM combinations due to the increased abrasion resistance provided by the hard ceramic materials. Differences in material stiffness, especially in COM combinations, have been shown to facilitate bearing lubrication with resulting decreased wear. Additionally, COM bearing combinations have a relatively high hardness differential ranging from about 3× to about 5×, and typically on the order of 4×, which is thought to contribute to lower wear rates.
Despite the low wear properties of COM and COC prostheses, ceramic heads present a risk of fracture, a risk that may steer some surgeons away from COM prostheses, despite the benefits of low wear. In addition, the brittleness and lower toughness of ceramic materials make it difficult to manufacture large diameter femoral heads of the type used in resurfacing procedures. MOM prostheses produce less wear particles than conventional metal-on-polymer (“MOP”) prostheses. MOM joint prostheses are typically made of cobalt based alloys conforming to ASTM F75 and/or F1537 specifications. In conventional MOM joint prostheses, the opposing bearing surfaces are made of the same cobalt chrome alloy and therefore have substantially the same hardness (typically, ranging from about 25 to 45 Rc). Thus far, little attention has been paid to the hardness of the MOM bearing surfaces. Factors other than hardness were thought to have a greater effect on wear rate in MOM hip prostheses. The most important wear factors are surface finish, clearance, and sphericity. Recent efforts to improve the performance of MOM joint prostheses have therefore focused on improving surface finish, clearance, and sphericity, rather than on the hardnesses of the cobalt chrome bearing surfaces.
Although MOM prostheses have lower wear volumes than MOP prostheses, metal wear particles are very small and high in number, and the physiological effect of metal wear particles is not fully understood. There is thus an interest in further reducing the volume of wear particles from MOM prostheses.
In ceramic-on-metal (“COM”) joint prostheses, the ceramic bearing surface is significantly harder than the metal bearing surface, and therefore necessarily provide a hybrid bearing effect. WO 0117464A1 (Fisher et al.) discusses improved wear in orthopedic prostheses through the use of COM. WO 0117464A1 disclosed that materials of the two surfaces can be selected with hardnesses that are greater than those in other joint systems so that the tendency for them to wear during articulation is reduced, and with a differential hardness which can ensure that one of the surfaces is generally able to remain smooth during articulation. This in turn can result in low wear of the opposite surface. WO 0117464A1 noted that the use of a ceramic material that is significantly harder than the metal material has the advantage that the tendency of the ceramic material to wear during articulation is minimized. WO 0117464 included wear testing data showing that wear debris from a COM prosthesis was significantly less than from a MOM prosthesis. According to WO 0117464, the MOM prosthesis showed a bedding-in wear rate of 3.12±0.45 mm3/106 cycles for about the first million cycles, which settled down to a steady state wear rate of 1.56±0.78 mm3/106 cycles. In contrast, the COM prosthesis showed essentially no bedding-in phase and a steady state rate of about 0.01 mm3/106 cycles over the course of a 3 million cycle test. Substantially all of the wear debris from the COM components was metal.
Further data comparing COM and MOM hip prostheses is provided in A Novel Low Wearing Differential Hardness, Ceramic-On-Metal Hip Joint Prosthesis, 34 J'l of Biomechanics, 1291-1298 (2001) (Firkins et al.). The Firkins article reported wear rates for MOM prostheses that showed a 100 fold higher degree of wear than for COM prostheses. Id. at 1296. Firkins tested femoral heads manufactured from medical grade alumina (ISO 6474) and femoral heads manufactured from medical grade low carbon (less than 0.07 percent) wrought cobalt chrome alloy (ASTM F 1537). Id. at 1293. Firkins coupled the ceramic and cobalt chrome heads with acetabular cups manufactured from medical grade high carbon (greater than 0.2 percent) wrought cobalt chrome alloy (ASTM F1537). Id. at 1293. Firkins reported a bedding in rate for MOM prostheses of 3.09±0.46 mm3/106 cycles during the first million cycles and a steady state wear rate of 1.23±0.5 mm3/106 cycles. Id. at 1294. The overall wear rate for MOM prostheses during the test was 1.62 mm3/106 cycles. Id. at 1294. Firkins noted that about 70 percent of the wear on the MOM prostheses occurred on the low carbon cobalt chrome heads. Id. at 1294. In contrast with MOM prosthesis wear, Firkins reported a wear rate on the COM prostheses of 0.1 mm3/106 cycles during a five million cycle test. Id. at 1294. The results of Firkins thus suggest that MOM prostheses wear at a rate 100 times greater than that of COM prostheses. Id. at 1294-96.
European Patent Application 0,841,041 A2 (Farrar) reported improved wear if the two articulating surfaces of a metal-on-metal prosthesis are formed from metals which are mismatched with respect to their carbon content. According to EP 0,841,041A2, testing of MOM hip prostheses having mis-matched carbon contents demonstrated that the lowest average wear (weight loss) was observed for prostheses in which a low carbon content alloy was used for the femoral head and a high carbon content alloy was used for the acetabular cup, or vice versa. (Col. 4, lines 46-53). EP 0,841,041A2 reported that the highest average wear was observed for prostheses in which the femoral head and acetabular cup were both formed from low carbon content alloy or both formed from high carbon content alloy. (Col. 4, lines 46-53). Although EP 0,841,041A2 reported testing of MOM prostheses for up to two million cycles, the patent failed to report actual wear test data, and it therefore impossible to quantitatively evaluate the wear claims made in the patent.
EP 0,841,041A2 did not note a difference in hardness between the mismatched carbons. This is probably because CoCrMo alloys with low carbon contents of different carbon contents have essentially the same hardness values. For example, a CoCr with a low carbon content of 0.07 percent-by-weight can have a hardness of 41 Rc, while a CoCr with a high carbon content of 0.25 percent-by-weight can have a hardness of 42 Rc.
In the general field of metal bearings, hybrid bearings consisting of a hard metal bearing in combination with a soft metal bearing have long been used to reduce wear of the bearing components. However, the hardness differential of hybrid bearings is provided by forming the bearing surfaces from two different types of metal, rather than from the same type of metal.
Despite prior art COM prostheses that by default have differential hardness bearing surfaces due to the use of different materials for the two bearing surfaces, no attempt has been made to explore the extent to which differential hardness concepts apply to the wear of MOM prostheses. There is thus a need for a joint prosthesis having the following characteristics and advantages over the prior art.